1. Field of the Invention
The present invention relates to a method of forming a semiconductor film comprising silicon or silicon compound having crystallinity over an insulating surface.
2. Description of the Related Art
A conventional technique is known in which a silicon film is formed over a glass or quartz substrate and a thin-film transistor (hereinafter referred to as “TFT”) is formed by using the thus-formed silicon film.
The TFT is mainly used in the active matrix liquid crystal display device. The TFT is generally classified into the TFT using an amorphous silicon film and the TFT using a crystalline silicon film.
At present, the TFT using an amorphous silicon film is the mainstream. However, the TFT using an amorphous silicon film is low in operation speed and hence its applicability is limited for purposes of reducing the size of a displayed image and displaying a high-speed moving picture.
Further, it is also attempted to constitute, by using TFTs, various circuits that are conventionally implemented as ICs. In this case, the operation speed of the TFT using an amorphous silicon film is much lower than a required value.
In view of the above, the TFT using a crystalline silicon film which is expected to operate at higher speed is now being studied extensively.
Among well known methods for obtaining a crystalline silicon film are:
(1) forming a crystalline silicon film directly by CVD or the like;
(2) crystallizing an amorphous silicon film by a heat treatment;
(3) crystallizing an amorphous silicon film by irradiating it with laser light; and
(4) crystallizing an amorphous silicon film by irradiating it with strong light such as infrared light.
Among the above methods, methods (2)-(4) are mainly used.
Although method (2) is advantageous in that it can easily provide a large-area film, the heat treatment temperature should be high and the quality of a resulting film is insufficient.
Although method (3) is advantageous in that thermal damage does not reach a glass substrate and a film having superior crystallinity can be obtained, it is difficult for method (3) to provide a large-area film and the reproducibility of a process is low.
Although method (4) can easily provide a large-area film, the quality of a resulting film is insufficient.
Studies of the present inventors revealed that the crystallization of an amorphous film can be accelerated by using a metal element typified by nickel (refer to Japanese Unexamined Patent Publication Nos. Hei. 6-232059 and Hei. 7-321339).
By combining the crystallization technique using a metal element with methods (2)-(4), a crystalline silicon film can be obtained that has such high film quality as could not be obtained so far.
However, the crystallinity thus obtained is still insufficient as compared to that of a single crystal silicon wafer and the characteristics of a resulting TFT are far lower than those of a currently available insulated-gate field-effect transistor that constitutes an IC. In particular, there is a serious problem that variations in device characteristics are large.
This is because grain boundaries exist in an uncontrollable state in a crystalline silicon film, i.e., in the channel of a TFT. In particular, since the grain boundaries extending direction cannot be controlled, the device characteristics vary to a large extent due to differences in extending directions of grain boundaries existing in the channels, which necessarily occur when a large number of devices are formed.